Phase failure responsive protection circuits for three-phase motors have been provided in which the absence of operativeness of all of the phases has been detected and upon such detection a means operated to disconnect the motor circuit. Means have also been provided in conjunction with this phase failure detection for detecting a current overload to supply a signal which also acts to operate the means for breaking the motor circuit. Thus this basic circuit has provided both overload protection and phase failure protection by the operation of the same means for breaking the motor circuit. Also circuitry has been provided for sensing overload conditions in an electric motor by means of current sensing loops in transformers which develop DC voltages. These DC voltages in turn are supplied to means for triggering a relay which upon operation will open a switch controlling the motor. In this device there is provided a limited adjustment making it possible for different magnitudes of current flowing in the motor windings to cause an overload tripping of the device and an interruption in the supply of the current to thus stop operation of the motor.
These prior circuits for motor controls are less than completely satisfactory. Their characteristics vary with frequency requiring calibration of the particular protection system to a specific disclosed frequency. The time for developing the trigger signal is not adjustable.
The adaptation of the protection system to a variety of frequencies, however, is important, particularly in certain motor uses. In drawing synthetic filaments, for example, the requirements placed on an individual power source can widely vary when providing the drive to conventional operations of the filament drawing machine. Moreover, this variation in requirements may be demanded of the individual power source within a short time span so that if a protection system is limited, as for example to a specific frequency, the incessant recalibration is onerous. What is desired is a three-phase motor overload protection device which cuts off the motor if the motor current exceeds the preset level for a time, the period of which is adjustable, for a range of motor currents over a base rating, the time of cutoff being related to the percent of overload.
There are other desired features in a motor protection circuit which are not provided by presently known systems. For example, it is advantageous to provide a means for detecting the condition where one of the phases in the three-phase motor is open and would be able to trip or trigger the means for disconnecting the motor current immediately if the motor current is operating at full load but at a somewhat longer period than the fastest trip time if the motor is operating partially loaded at a current value which is below the full load rating. Under a condition at motor start-up where one of the windings is open and non-conductive it is desirable to limit the voltage output from the current transformer during the period of conduction under this faulty condition by utilizing some means for limiting this voltage output from the current transformer.
It is also desirable to provide protection against operation when less than all of the phases are carrying current both during the running of the motor and when attempting to start the motor, as for example on a single phase and also to protect the motor windings from excessive current conditions and to provide this protection by operating the same voltage sensitive circuit the same way by switching on a transistor by producing a negative resistance in the transistor upon creation of a forward current.
It is also desirable to provide protection for components other than the motor and that the protection system provide this additional feature.
It is an object of this invention to provide a three-phase motor protection device which is adapted to operating at different frequencies and voltages and over a wide range of motor currents for triggering the protective action.
It is another object of this invention to provide for control of the motor power which varies the time function of the control inversely to the degree of current increase.